The Prime Minister said he was “very concerned” about the threat from North Korea and warned that Pyongyang now has capabilities to strike the United States and Britain.

Speaking to workers in Scotland the Prime Minister said the threat from North Korea showed that Britain needs to keep the Trident nuclear deterrent.

Asked if he was concerned about North Korea the Prime Minister said: “How concerned am I about North Korea? Very concerned, it has extremely dangerous technologies in terms of nuclear and its weapons.”

However, he added: “The fact is, as I wrote in a newspaper article this morning, North Korea does now have missile technology that is able to reach, as they put it, the whole of the United States and if they’re able to reach the whole of the United States they can reach Europe too. They can reach us too, so that is a real concern.”

North Korea is not thought to have nuclear-armed missiles that would be capable of hitting the United States or the UK, so the Prime Minister’s latest comments are likely to cause alarm.

Mr Cameron’s comments are significantly more definitive than words he used in an article for The Daily Telegraph this morning, when he said: “Last year North Korea unveiled a long-range ballistic missile which it claims can reach the whole of the United States.

“If this became a reality it would also affect the whole of Europe, including the UK.”

It is not yet known whether Mr Cameron misspoke during his speech to Scottish workers or whether he was disclosing new information about an increased threat.

Mr Cameron added: “[North Korea] has a new and relatively unknown leader and obviously the noises it’s been making in recent weeks and months are worrying and threatening.

“What matters is that North Korea should abide by all the United Nations resolutions laid down. We do need to make sure this whole situation - that the heat is taken out of it. But it is almost entirely North Korea that is able to do that and ought to do that.

“But I think it’s a good moment to stand back and ask ourselves about the dangers there are in the world and the need to maintain strong defences.”